1. Field of the Invention
This invention relates to synthetic ester base functional fluid compositions which are useful as jet engine lubricants, heat transfer fluids, hydraulic fluids and the like, especially at high temperatures. More specifically, the present invention relates to synthetic ester base stocks which have achieved improved oxidative stability through the incorporation of triphenylphosphine or triphenylphosphine oxide together with a complex alkali metal organophosphorus compound.
2. Description of Prior Art
The continuing development and improvement of high performance aircraft gas turbine engines has created a continual demand for improved lubricants which are efficient over a wide range of operating temperatures and which are resistant to oxidative and thermal degradation. Aircraft gas turbine engines in particular are found to operate with greater efficiency at higher gas temperatures. Higher gas temperatures necessarily result in higher lubricant temperatures in the hot section of the engine. Consequently, emphasis on improved gas turbine lubricants has usually been directed toward their high temperature properties.
The special requirements of gas turbines have led to the development of synthetic lubricants, the most prevalent being polyesters formulated from hindered alcohols such as pentaerythritol, dipentaerythritol and mixtures thereof. These synthetic esters usually form the lubricant base stock to which various compounds are added to enhance and control such properties as viscosity, corrosivity, oxidation stability, foaming, flammability, extreme pressure properties and loadbearing capability.
Oxidative stability of the synthetic esters is reflected in the change in viscosity and corrosivity of the material after exposure to air at elevated operating temperatures. Typically, uninhibited lubricant formulations show large increases in both viscosity and corrosivity as a result of oxidation.
Numerous oxidation and corrosion inhibitors have been found useful in ester base lubricating compositions. For example, antioxidants such as phenothiazine, phenyl-1-naphthylamine, diaromatic secondary amines, and mixtures of alkali metal salts of carboxylic acids with arylamines are well known in the art. Many stabilizers, however, have one or more deficiencies which limit their use and make them unsatisfactory for current needs. Quite often, these inhibitors either do not provide adequate oxidation resistance for modern high performance, high temperature gas turbines, or they adversely affect other critical properties such as load-bearing capacity.
Alkali metal salts are known to inhibit high temperature oxidation of organic fluids. Those salts which are soluble enough in ester base stocks to be effective often tend to cause excessive foaming and sometimes react with the esters to form insoluble salts.
Illustrative of the steadily increasing demands being placed upon gas turbine engine lubricants is the upward progression of bulk oil temperature limits in Military Specifications as follows:
______________________________________ Specification MIL-L-7808 -54.degree. C. to +163.degree. C. Specification MIL-L-23699 -40.degree. C. to +177.degree. C. Specification MIL-L-27502 -40.degree. C. to +240.degree. C. ______________________________________
Specification MIL-L-27502 is applicable to synthetic lubricants visualized for advanced gas turbine engine designs. Synthetic fluids now available under the earlier specifications are known to experience excessive deposit formation and degradation in the range of 177.degree. C. to 240.degree. C. Development of a formulated ester base lubricant meeting the rigorous demands of MIL-L-27502 was an object of the present invention.
U.S. Pat. No. 3,684,711, issued Aug. 15, 1972, discloses a deposit inhibitor for synthetic ester base lubricants. That deposit inhibitor comprised a complex alkali metal organophosphorus compound prepared by esterifying a polyhydric alcohol free of beta hydrogen with a carboxylic acid or acid derivative and a phosphorus acid or acid derivative. The resulting complex organophosphorus ester was thereafter contacted with an alkali metal base.
It is an object of the present invention to provide certain oxidation inhibitors which cooperate with the deposit inhibitors of U.S. Pat. No. 3,684,711 in an ester base stock to produce a gas turbine engine lubricant capable of operation at bulk oil temperatures of 240.degree. C. without excessive formation of engine deposits and without excessive viscosity increase. A further object of the present invention is to provide an oxidatively stable synthetic lubricant as described above which exhibits satisfactory storage life. Still another object of the present invention is to provide inhibitors for esterbase lubricants which do not adversely affect the load-bearing properties of the lubricants. Still another object is to provide superior lubricants for modern high performance, high temperature gas turbine engines.